Safety of acid-injection mining technique debated

by Ryan Randazzo - Sept. 8, 2012 11:48 PMThe Republic | azcentral.com

The tantalizing copper deposit at the foot of a butte near Florence has drawn interest from mining firms since the 1960s. But the cost of extracting the rich ore from deep underground always sent miners in this copper-rich state to more easily harvested deposits.

Rising copper prices have changed the equation, prompting Curis Resources to embark on a plan to extract the copper by injecting sulfuric acid into formations more than 400 feet underground, where it would strip the fractured rock of copper. The mix of acid and minerals would be pumped back to the surface for processing.

The process, called "in-situ" mining, meaning "in place," is a point of white-hot controversy in a battle between the mining company and Southwest Value Partners, a developer seeking approval for a nearby housing project.

The key question about Curis' proposal is whether pumps could recapture all of the acid pumped underground, where it could encounter groundwater, geologic fault lines, abandoned mining tunnels and nearby wells.

Opponents, including Southwest, fear it would poison the water supply and derail the planned development. But mining experts say the technique is safe and effective.

The mine's future turns on securing permits from the U.S. Environmental Protection Agency and the Arizona Department of Environmental Quality. The agencies are expected to make decisions later this year on whether the mining operation would be safe.

Tunnels and rooftops

The ore deposit lies beneath about 200 acres of agricultural land -- half belonging to Florence, the other the state trust.

After being bought and sold by various mining companies, Merrill Ranch Development acquired the site in 2001.

It was annexed by Florence and zoned for houses and businesses.

Merrill went bankrupt and Southwest and other investors purchased surrounding parcels, not anticipating the mine would restart.

In addition to Southwest and other nearby landowners, Johnson Utilities, which provides the region with water, also has become a vocal mine opponent, concerned it could pollute wells.

In November, the Florence Town Council rejected zoning changes required to mine half the ore deposit. Curis is proceeding with a test project and mine on the state-owned portion, hoping that the full deposit can one day be mined.

'Hydraulic control'

Traditionally, miners remove copper-bearing ore and either send it to a smelter to be processed, or stack it in "heap-leach" piles that are sprayed with acid. As with the in-situ mine that Curis proposes, acid dissolves the copper and is collected. A process called electrowinning then removes the copper from the solution.

In-situ mining is a less expensive process because it saves the energy, equipment, time and labor needed to dig hundreds of feet into bedrock.

A key question being examined by EPA and ADEQ is "hydraulic control," or whether miners can recapture all of the acid injected underground.

The Florence copper lies in a section of bedrock saturated with groundwater.

There is no question acid would render groundwater under Curis land undrinkable while the mine was pumping acid. Cleaning up the site would continue for years after the 20 or so years the mine is projected to operate.

What is debated is whether Curis could limit the damage to the water directly beneath its property and recapture it with pumps, or whether it would affect neighbors drawing groundwater.

Adding sulfuric acid to groundwater also would dissolve naturally occurring sulfate and radioactive material in the bedrock, which opponents worry could seep beyond the site.

In high enough concentrations, sulfate gives water a bad taste and odor and can upset the stomachs of people unaccustomed to drinking it. Radioactive chemicals such as uranium can cause kidney damage and cancer.

Mine officials said pumps would prevent those elements from leaving the site. Once mining is complete, the company would inject water into the aquifer to "rinse" it. That would dilute the radiochemicals to their background levels, said Dan Johnson, Curis' vice president and general manager for environmental and technical services.

He said they could maintain hydraulic control of acid underground by pumping more fluid to the surface than is injected.

"The fluid will always go toward your well. You can't defy gravity. That is the premise of it," he said.

He said the groundwater would be protected even if one or more of the recovery pumps failed.

Opponents are not convinced.

"Our concern is: How do you inject all of this acid into the the fractured rock, with all of the boreholes and other potential conduits, and still protect the aquifer?" said Kevin Hebert, a geologist with Southwest Ground-water Consultants Inc. in Phoenix, hired by mine opponents.

"It's all these fractures and communication between the aquifers that scares everybody."

Two key concerns

Southwest Value and Florence plan several wells and a water-treatment facility directly west of the mine. Opponents say those wells likely would draw contaminants from the Curis land.

Groundwater in the area generally flows northwest, but that can change when new wells begin draining the aquifer, said Hebert.

Some planned wells would be less than 1,000 feet from the mine, he said.

Curis officials counter that the state would prohibit wells so close to the project, and that groundwater west of their land is set aside for agriculture.

"The idea that Southwest Value Partners is going to come along and drill wells on the west end of the property is just wrong," said Rita Maguire, a former Arizona Department of Water Resources director now working for Curis.

Florence commissioned a report from hydrologists at Tucson's Montgomery and Associates to study the effects on the town's water.

The town is south of the mine site, upstream in the flow of groundwater. Montgomery and Associates determined that if the mine were built and run properly, and if Florence was careful about where it built new wells, "the proposed (mine) could be implemented in a manner that is consistent with the goals of the town of Florence general plan."

But Montgomery officials also said new wells west of the property -- exactly where Southwest Value plans them -- could draw groundwater polluted by the mine.

Essentially, drinking-water pumps near the mine could overpower Curis' acid-recovery pumps, sucking polluted groundwater from under the mine site.

Experts supportive

Experts say that while the in-situ technique sounds intimidating to lay people, the technology is proven.

"There are technical solutions to their concerns," said J. Brent Hiskey, a University of Arizona professor of mining and geological engineering. "This technique has a very high degree of certainty. Over time, a long time, this would totally be remediated."

Carl Nesbitt, an in-situ mining expert at the University of Nevada, Reno, agreed.

"(In-situ mines) never want anything to get away," he said. "They take a lot of precautions to make sure the solutions don't get into anything or hurt anything. No. 1, it is environmental, but No. 2, that is lost profit."

He said the solutions are easy to control with pumps. He compared the process to a bathtub, drawing the water down with gravity toward extraction pumps in a controlled way.

"That is what these companies do," he said. "They build very nice bathtubs in the middle of the desert."

A track record

Yet disagreement continues on the impact of in-situ test mining at the site that ended more than 14 years ago. The site has been monitored almost constantly since.

Curis officials point to approvals from the state and EPA that allowed the site's previous owner, BHP, to inject acid into the site from October 1997 to February 1998.

"This field test demonstrated BHP maintained hydraulic control and that the in-situ leaching system works as designed," BHP Project Manager John Pio wrote at the time to the EPA.

But Hebert said BHP's data shows it did not always control the fluids. Compliance reports submitted to the EPA from Nov. 8, 1997, show on that day, groundwater was flowing away from a recovery well intended to collect the acid.

Monitoring wells at the Florence site occasionally turn up an "exceedance" of sulfate and other minerals. Companies aren't fined for exceedances, but a pattern can indicate violations of groundwater-protection rules.

Curis and its opponents debate whether recent exceedances are caused by the small amount of acid pumped underground 14 years ago or natural fluctuations in groundwater dissolving minerals in the bedrock.

Other locations

The track record of other in-situ mines in Arizona has only contributed to the debate.

Magma Copper Co. began in-situ mining at Pinal County's San Manuel mine in 1987, using 2 billion gallons of acid to treat ore beneath the open pit mine, according to EPA records.

An EPA report from May 1992 indicates that Magma, for an unknown amount of time, pumped 8,000 gallons a minute of acid into the mine, but only recovered 7,200 gallons a minute. That meant 800 gallons per minute was escaping into the bedrock until engineers re-established control.

Johnson of Curis said the San Manuel project was different from Curis' proposal because miners operated in bedrock that was pumped dry to dig the pits, making it hard to control the fluid with pumps.